Electrode for lithium secondary cell and lithium secondary cell

ABSTRACT

An electrode having a current collector and, formed thereon, a thin film comprising an active material, characterized in that a thin alloy film (such as Sn—Co) comprising a metal which can form an alloy with lithium (such as Sn) and a metal which can not form an alloy with lithium (such as Co) is formed on a current collector such as a copper foil. It is preferred that the above metal which can form an alloy with lithium and the above metal which can not form an alloy with lithium can not form an intermetallic compound with each other.

TECHNICAL FIELD

[0001] The present invention relates to a novel electrode for arechargeable lithium battery and also to a rechargeable lithium batteryutilizing the same.

BACKGROUND ART

[0002] Rechargeable lithium batteries, recently under extensivedevelopment and research, exhibit battery characteristics, such ascharge-discharge voltage, charge-discharge cycle life characteristicsand storage characteristics, which depend largely upon the types of theelectrodes used. This has led to the various attempts to better batterycharacteristics by improving active electrode materials.

[0003] The use of metallic lithium as the negative active materialenables construction of batteries which exhibit high energy densities,both gravimetric and volumetric. However, the lithium deposited oncharge grows into dendrites, which could cause problematic internalshort-circuiting.

[0004] On the other hand, rechargeable lithium batteries are reportedusing an electrode composed of aluminum, silicon, tin or the like whichalloys electrochemically with lithium during charge (Solid State Ionics,113-115, p57 (1998)).

[0005] However, such lithium-alloying materials when used as thenegative electrode material undergo large volumetric expansion andshrinkage as they store and release lithium. The subsequentpulverization and separation thereof from the current collector makes itdifficult to obtain satisfactory charge-discharge cycle characteristics,which has been a problem.

DISCLOSURE OF THE INVENTION

[0006] It is an object of the present invention to provide an electrodefor a rechargeable lithium battery, which exhibits a high dischargecapacity and superior charge-discharge characteristics, as well asproviding a rechargeable lithium battery using the same.

[0007] The electrode for a rechargeable lithium battery, in accordancewith the present invention, is characterized as comprising a currentcollector and an alloy thin film provided on the current collector andcomposed of a metal that alloys with lithium and a metal that does notalloy with lithium.

[0008] In the present invention, the metal that alloys with lithiumrefers to a metal which forms an alloy, such as a solid solution orintermetallic compound, with lithium. Specific examples of suchlithium-alloying metals include Sn, Ge, Al, In, Mg, Si and the like.

[0009] In the present invention, the metal that does not alloy withlithium refers to a metal which does not form an alloy, such as a solidsolution or intermetallic compound, with lithium and more specificallyto a metal which does not show the presence of an alloy state in itsbinary phase diagram with lithium. Examples of such nonlithium-alloyingmetals include Cu, Fe, Ni, Co, Mo, W, Ta, Mn and the like.

[0010] In the present invention, the nonlithium-alloying metal ispreferably of the type that forms an intermetallic compound with thelithium-alloying metal. The intermetallic compound, as used herein,refers to a compound which has a specific crystal structure containingmetals in a specific proportion. In the present invention, if thelithium-alloying metal is Sn, the nonlithium-alloying metal ispreferably of the type that forms an intermetallic compound with Sn.Such a nonlithium-alloying metal may contain at least one metal selectedfrom Ti, Mn, Fe, Ni, Co, Cu, Zr and Mo. One or more of these metals maybe contained. Preferred among those metals is at least one selected fromFe, Co and Ni. Particularly preferred is Co. In the present invention,the thin alloy film composed of such metals does not necessarily containan intermetallic compound thereof. The thin alloy film is notnecessarily crystalline and may be amorphous or comprise anonstoichiometric compound.

[0011] In the present invention, the thin alloy film composed of thelithium-alloying metal and the nonlithium-alloying metal is provided onthe current collector. Although not limiting, formation of the thinalloy film is preferably achieved by an electrochemical process such aselectrolytic plating and electroless plating. Physical thin film-formingprocesses, such as CVD, sputtering, vapor evaporation and thermalspraying, can also be utilized to form the thin alloy film.

[0012] The current collector for use in the present invention is notparticularly specified, so long as it is applicable for use in anelectrode for a rechargeable lithium battery. The current collector maycomprise a metal foil composed of copper, nickel, titanium, iron,stainless steel, molybdenum, cobalt, chromium, tungsten, tantalum,silver or the like, for example.

[0013] Preferably, the current collector for use in the presentinvention has irregularities on its surface. An upper limit of surfaceroughness Ra of the current collector is not particularly specified.However, in general, a copper foil having a practical thickness forbatteries and a surface roughness Ra of exceeding 2 μm is not readilyavailable in the market. Under such circumstances, the upper limit ofsurface roughness Ra is preferably 2 μm or below, more preferably 1 μmor below. On the other hand, a lower limit of surface roughness Ra ispreferably 0.01 μm or above. Accordingly, the surface roughness Ra ispreferably in the range of 0.01-2 μm, more preferably in the range of0.01-1 μm.

[0014] The surface roughness Ra is defined in Japan Industrial Standards(JIS B 0601-1994) and can be determined as by a surface roughness meter.When a copper foil having a large surface roughness Ra is desired for acurrent collector, the use of an electrolytic copper foil is preferred.

[0015] In the present invention, it is preferred that the thin alloyfilm is separated into islands by gaps or spaces formed therein in amanner to extend in its thickness direction. If the thin alloy film isseparated into islands while it remains adherent to the currentcollector, a marked improvement of charge-discharge cyclecharacteristics results.

[0016] Because of inclusion of the lithium-alloying metal, the thinalloy film can store lithium via alloying therewith during acharge-discharge reaction. For example, in the case where the electrodeof the present invention is used as a negative electrode, the thin alloyfilm stores lithium during charge and releases lithium during discharge.As the thin alloy film stores and releases lithium in such a fashion, itexpands and shrinks in volume. The separation of the thin alloy filminto islands results in the provision of spaces that surround theislands. These surrounding spaces can accommodate changes in volume ofthe thin alloy film as it expands and shrinks during charge-dischargecycles. Accordingly, no strain is produced in the thin alloy film. Thisprevents pulverization and separation thereof from the currentcollector.

[0017] The plating or physical thin film-forming process involvesdepositing, in the form of a continuous thin film, an alloy onto thecurrent collector. If this is the case, the first or subsequentcharge-discharge reaction causes formation of the gaps which extend inthe thickness direction of the thin alloy film. When the thin alloy filmexpands and subsequently shrinks in the charge-discharge reaction, suchgaps are formed along the thickness direction to separate the thin alloyfilm into islands. Such separation of the thin alloy film along the gapsextending in its thickness direction is facilitated particularly whenthe current collector having surface irregularities is used. Depositionof the thin alloy film on the current collector having surfaceirregularities results in the formation of the corresponding surfaceirregularities on a surface of the deposited thin alloy film. It isbelieved that when such a thin alloy film expands and shrinks, gaps areformed along lines which extend between respective valleys of theirregularities on the thin alloy film surface and on the currentcollector surface, so that the thin alloy film is separated into islandsalong the valleys of the irregularities on the current collectorsurface.

[0018] In the present invention, the thin alloy film preferably containsup to 50% of the nonlithium-alloying metal, based on a molar ratio(atomic ratio). If the content exceeds this range, the relative amountof the lithium-alloying metal becomes small to result in the undesirablereduction of a charge-discharge capacity. It is also preferred that thethin alloy film contains at least 0.1% of the nonlithium-alloying metal,based on a molar ratio (atomic ratio). Inclusion of nonlithium-alloyingmetal reduces volumetric expansion and shrinkage of the thin alloy filmduring a charge-discharge reaction to result in the improvedcharge-discharge cycle characteristics. In view of this cyclecharacteristic improvement, it is preferred that the thin alloy filmcontains at least 0.1% of the nonlithium-alloying metal. Hence, thenonlithium-alloying metal is preferably incorporated in the thin alloyfilm within the range of 0.1-50%, based on a molar ratio (weight ratio),more preferably 1-40%, based on a molar ratio (atomic ratio).

[0019] In the present invention, a mixed layer of components of thecurrent collector and the alloy may be formed at an interface betweenthe current collector and the thin alloy film. Formation of such a mixedlayer assures better adhesion of the thin alloy film to the currentcollector, so that further improvement of cycle characteristics can beexpected. This mixed layer can be formed, for example, by depositing thethin alloy film on the current collector and then subjecting thedeposited thin alloy film to a heat treatment or the like. Preferably,the heat treatment is carried out at a temperature lower than therespective melting points of the thin alloy film and the currentcollector.

[0020] The rechargeable lithium battery of the present invention ischaracterized as including a negative electrode comprised of theelectrode of the present invention for a rechargeable lithium battery, apositive electrode and a nonaqueous electrolyte.

[0021] An electrolyte solvent for use in the rechargeable lithiumbattery of the present invention is not particularly specified in typebut can be illustrated by a mixed solvent which contains cycliccarbonate such as ethylene carbonate, propylene carbonate, butylenecarbonate or vinylene carbonate and also contains chain carbonate suchas dimethyl carbonate, methyl ethyl carbonate or diethyl carbonate. Alsoapplicable is a mixed solvent of the above-listed cyclic carbonate andan ether solvent such as 1,2-dimethoxyethane or 1,2-diethoxyethane.Examples of electrolyte solutes include LiPF₆, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃ and mixtures thereof. Illustrative of other applicableelectrolytes are gelled polymer electrolytes comprised of an electrolytesolution impregnated into polymer electrolytes such as polyethyleneoxide and polyacrylonitrile, and inorganic solid electrolytes such asLiI and Li₃N, for example. The electrolyte for the rechargeable lithiumbattery of the present invention can be used without limitation, so longas an Li compound as its solute that imparts an ionic conductivity,together with its solvent that dissolves and retains the Li compound,remain undecomposed at voltages during charge, discharge and storage ofthe battery.

[0022] Examples of useful active materials of the positive electrode forthe rechargeable lithium battery of the present invention includelithium-containing transition metal oxides such as LiCoO₂, LiNiO₂,LiMn₂O₄, LiMnO₂, LiCo_(0.5)Ni_(0.5)O₂ and LiNi_(0.7)Co_(0.2)Mn_(0.1)O₂;and lithium-free metal oxides such as MnO₂. Other substances can also beused, without limitation, if they are capable of electrochemical lithiuminsertion and deinsertion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a photomicrograph taken using a scanning electronmicroscope, showing a surface of the electrode a1 in accordance with anembodiment of the present invention;

[0024]FIG. 2 is a photomicrograph taken using a scanning electronmicroscope, showing a section of the electrode a1 in accordance with theembodiment of the present invention;

[0025]FIG. 3 is a schematic sectional view, showing a beaker cellconstructed in one example;

[0026]FIG. 4 is a plan view, showing a rechargeable lithium batteryconstructed in one example; and

[0027]FIG. 5 is a sectional view, showing a combination of electrodes inthe rechargeable lithium battery shown in FIG. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

[0028] The present invention is below described in more detail by way ofExamples. It will be recognized that the following examples merelyillustrate the practice of the present invention but are not intended tobe limiting thereof. Suitable changes and modifications can be effectedwithout departing from the scope of the present invention.

[0029] Experiment 1

[0030] (Fabrication of Electrodes)

[0031] An electrolytic plating technique was utilized to deposit, in theform of a thin film having a thickness of 2 μm, an Sn—Co alloy on anelectrolytic copper foil (surface roughness Ra=0.188 μm). A plating bathwas used containing a mixture of tin chloride, cobalt chloride, sodiumchloride, hydrochloric acid, ethylene glycol and thiourea.

[0032] After deposition of the Sn—Co alloy thin film, the stack was cutinto a size of 2 cm×2 cm to provide an electrode a1.

[0033] For a comparative purpose, a slurry was prepared containing amixture of an Sn—Co alloy powder (molar ratio of 8:2) produced viaatomization and a fluoro resin (PVdF) at a 95:5 ratio by weight, theslurry was coated onto an electrolytic copper foil and then dried, andthe resulting stack was cut into a size of 2 cm×2 cm to provide anelectrode b1.

[0034] (Preparation of Electrolyte Solution)

[0035] 1 mole/liter of LiPF₆ was dissolved in a mixed solvent containingethylene carbonate and diethyl carbonate at a 1:1 ratio by volume toprepare an electrolyte solution.

[0036] (Construction of Beaker Cell)

[0037] Using each of the above-fabricated electrodes a1 and b1 as aworking electrode, a beaker cell shown in FIG. 3 was constructed. Asshown in FIG. 3, the beaker cell includes a counter electrode 3, aworking electrode 4 and a reference electrode 5, which are all immersedin an electrolyte solution contained in a container 1. Theabove-prepared electrolyte solution was used as the electrolyte solution2. Metallic lithium was used for both the counter electrode 3 and thereference electrode 5.

[0038] (Measurement of Charge-Discharge Characteristics)

[0039] The above-constructed beaker cell was charged at 25° C. at aconstant current of 0.2 mA to 0 V (vs. Li/Li⁺) and then discharged at aconstant current of 0.2 mA to 2 V (vs. Li/Li⁺). This unit cycle ofcharge and discharge was repeated 10 times to measure a charge capacityand a discharge capacity per gram of active material on each cycle andthen calculate an initial efficiency and a capacity retention rate aseach defined below. The results are given in Table 1. In this beakercell, reduction of the working electrode takes place during charge andoxidation thereof takes place during discharge.

Initial efficiency (%)=(1st-cycle discharge capacity/1st-cycle chargecapacity)×100

Capacity retention rate (%)=(10th-cycle discharge capacity/1st-cycledischarge capacity)×100 TABLE 1 1st-Cycle 1st-Cycle 10th-Cycle CapacityCharge Discharge Initial Discharge Retention Capacity CapacityEfficiency Capacity Rate Electrode (mAh/g) (mAh/g) (%) (mAh/g) (%) a1772 632 82 628 99 b1 403 309 77 3 1

[0040] As apparent from the results shown in Table 1, the electrode a1in accordance with the present invention has the increased dischargecapacity and exhibits the improved cycle characteristics compared to thecomparative electrode b1.

[0041] Experiment 2

[0042] Rechargeable lithium batteries were constructed using theelectrodes a1 and b1 as their respective negative electrodes and thenevaluated for charge-discharge cycle characteristics.

[0043] (Fabrication of Positive Electrode)

[0044] 85% by weight of LiCoO₂ powder having a mean particle diameter of10 μm, 10% by weight of carbon powder as an electric conductor and 5% byweight of polyvinylidene fluoride as a binder were mixed together.N-methylpyrrolidone was added to the mixture which was subsequentlykneaded to prepare a slurry. This slurry was coated onto one surface ofa 20 μm thick aluminum foil using a doctor blade technique and thendried. The resulting stack was cut into a 2 cm×2 cm size to provide apositive electrode.

[0045] (Construction of Battery)

[0046] The above-fabricated positive electrode and electrode a1 or b1were bonded to each other with a microporous polyethylene film betweenthem. The resulting combination was inserted into an outer casing madeof an aluminum laminated material, and 500 μl of an electrolyte solutionsimilar in type to that prepared in Experiment 1 was further introducedinto the outer casing to thereby assemble a rechargeable lithiumbattery.

[0047]FIG. 4 is a plan view, showing the rechargeable lithium batteryconstructed. As shown in FIG. 4, a combination of the positive electrode11, the negative electrode 13 and the intervening separator 12comprising a polyethylene microporous film are inserted into the outercasing 14. After insertion of the combination into the outer casing 14,the electrolyte solution is introduced thereinto. A rechargeable lithiumbattery is constructed by sealing the outer casing 14 at its portion 14a.

[0048]FIG. 5 is a sectional view which shows how the electrodes arecombined within the battery. As shown in FIG. 5, the positive electrode11 and the negative electrode 13 are disposed on opposite sides of theseparator 12. The positive electrode 11 includes a positive currentcollector 11 b made of aluminum and a layer 11 a of positive activematerial which overlies the positive current collector and contacts withthe separator 12. Likewise, the negative electrode 13 includes anegative current collector 13 b made of copper and a layer 13 a ofnegative active material which overlies the negative current collectorand contacts with the separator 12. In this example, the layer 13 a ofnegative active material comprises an Sn—Co alloy thin film.

[0049] As shown in FIG. 4, an externally-extending aluminum positive tab11 c is attached to the positive current collector 11 b. Likewise, anexternally-extending nickel negative tab 13 c is attached to thenegative current collector 13 b.

[0050] The rechargeable lithium batteries constructed using theelectrodes a1 and b1 as their respective negative electrodes weredesignated as a battery A1 and a battery B1. A design capacity of eachbattery was 6 mAh.

[0051] (Charge-Discharge Test)

[0052] The above-constructed batteries A1 and B1 were subjected to acharge-discharge test. A unit cycle was designed to consist of asequence of charging at a constant current of 1.2 mA to a chargecapacity of 6 mAh and discharging at a constant current of 1.2 mA to 2.0V. Exceptionally, the 1st-cycle charge (on the first cycle) wascontinued to a charge capacity of 7.2 mAh. The procedure of the aboveExperiment 1 was followed to calculate the initial efficiency and thecapacity retention rate for those batteries. The results are given inTable 2. The measurement was carried out at 25° C. TABLE 2 1st-Cycle1st-Cycle 10th-Cycle Capacity Charge Discharge Initial DischargeRetention Capacity Capacity Efficiency Capacity Rate Battery (mAh/g)(mAh/g) (%) (mAh/g) (%) A1 7.2 5.9 82 4.8 81 B1 7.2 5.5 77 0.06 1

[0053] As apparent from the results shown in Table 2, the battery A1 inaccordance with the present invention exhibits superior charge-dischargecycle performance characteristics.

[0054]FIG. 1 is a photomicrograph taken using a scanning electronmicroscope at a magnification of 1,000×, showing a surface of theelectrode a1 which was taken out from the battery A1 after 10 cycles inthe charge-discharge test. FIG. 2 is a photomicrograph taken using ascanning electron microscope at a magnification of 5,000×, showing asection of the electrode a1 which was embedded in a resin and thensliced. As can be clearly seen from FIGS. 1 and 2, the thin alloy filmin the electrode a1 after a charge-discharge reaction is separated intoislands by gaps formed therein to extend in its thickness direction. Asapparent from FIG. 2, these gaps extend along valleys of irregularitieson a surface of the current collector. As also apparent from FIG. 1,these gaps are connected like a network along the valleys ofirregularities on the current collector surface, when viewed in a planeof the thin alloy film.

[0055] As can be clearly seen from FIG. 2, the thin alloy film isdeposited to run over and along the irregularities on the currentcollector surface, and the gaps are formed along lines which extendbetween respective valleys of the irregularities on the thin alloy filmsurface and on the current collector surface. It is believed thatexpansion and shrinkage of the thin alloy film during a charge-dischargereaction has caused formation of such gaps.

[0056] As shown in FIGS. 1 and 2, spaces exist to surround the islandsof the thin alloy film. These spaces are believed to accommodate changesin volume of the thin alloy film during the charge-discharge reactionand contribute to the improvement of cycle characteristics.

[0057] In the above example, the deposition of the thin Sn—Co alloy filmon a current collector substrate was achieved by an electrolytic platingprocess. Alternatively, an electroless plating process may be utilized.Other thin film-forming processes such as sputtering, vacuum depositionand thermal spraying can also be utilized.

[0058] Experiment 3

[0059] Analogous to Experiment 1, an electrolytic plating process wasutilized to deposit, in the form of a thin film having a thickness of 2μm, an Sn—Ni, Sn—Fe, Sn—Pb or Sn—Zn alloy onto an 18 μm thickelectrolytic copper foil (surface roughness Ra=0.188 μm).

[0060] Deposition of the Sn—Ni alloy thin film was effected using anSn—Ni plating bath containing a mixture of potassium pyrophosphate, tinchloride, nickel chloride and glycine.

[0061] Deposition of the Sn—Fe alloy thin film was effected using anSn—Fe plating bath containing a mixture of tin chloride, iron sulfate,sodium citrate and L-ascorbic acid. The Sn—Fe plating bath was used indifferent two compositions.

[0062] Deposition of the Sn—Pb alloy thin film was effected using anSn—Pb plating bath containing a mixture of tin borofluoride, leadborofluoride, fluoroboric acid, boric acid and peptone.

[0063] Deposition of the Sn—Zn alloy thin film was effected using anSn—Zn plating bath containing a mixture of organic tin, organic zinc anda complexing agent.

[0064] The electrode made via deposition of the Sn—Ni thin film wasdesignated as an electrode cl of the present invention. The electrodesmade via deposition of the Sn—Fe thin films having differingcompositions were designated as electrodes c2 and c3 of the presentinvention, respectively. The electrodes made via deposition of the Sn—Pband Sn—Zn thin films were designated as comparative electrodes e1 ande2, respectively. Ni and Fe are nonlithium-alloying metals. Sn, Pb andZn are lithium-alloying metals. Hence, the Sn—Ni alloy thin film and theSn—Fe alloy thin film fall within the scope of the present invention,but the Sn—Pb alloy thin film and the Sn—Zn alloy thin film fallsoutside the scope of the present invention.

[0065] The compositions of the thin alloy films incorporated in theelectrodes c1-c3 of the present invention and comparative electrodes e1and e2 were analyzed by ICP emission spectrometry. The composition ofeach thin alloy film is given in Table 3. In Table 3, the composition ofthe thin alloy film incorporated in the electrode a1 of the presentinvention in Experiment 1 is also shown. TABLE 3 Weight Ratio AtomicRatio Electrode (%) (%) Electrode a1 of this Invention 83Sn-17Co71Sn-29Co (Sn—Co) Electrode c1 of this Invention 81Sn-19Ni 68Sn-32Ni(Sn—Ni) Electrode c2 of this Invention 91Sn-9Fe 83Sn-17Fe (Sn—Fe)Electrode c3 of this Invention 86Sn-14Fe 74Sn-26Fe (Sn—Fe) ComparativeElectrode e1 82Sn-18Pb 89Sn-11Pb (Sn—Pb) Comparative Electrode e289Sn-11Zn 82Sn-18Zn (Sn—Zn)

[0066] Using each of the electrodes c1-c3 of the present invention andthe comparative electrodes e1 and e2 as a working electrode, a beakercell was constructed in the same manner as in Experiment 1, and thenevaluated for cycle characteristics. The evaluation results are listedin Table 4. TABLE 4 1st-Cycle 1st-Cycle 10th-Cycle Capacity ChargeDischarge Initial Discharge Retention Capacity Capacity EfficiencyCapacity Rate Electrode (mAh/g) (mAh/g) (%) (mAh/g) (%) Electrode 578550 95 519 95 c1 of this Invention (Sn—Ni) Electrode 626 574 92 427 74c2 of this Invention (Sn—Fe) Electrode 663 593 89 484 82 c3 of thisInvention (Sn—Fe) Compara- 704 649 92 156 24 tive Electrode e1 (Sn—Pb)Compara- 789 749 95 32 4 tive Electrode e2 (Sn—Zn)

[0067] As apparent from the results shown in Table 4, the electrodesc1-c3 of the present invention all exhibit the improved cyclecharacteristics compared to the comparative electrodes e1 and e2.

[0068] Experiment 4

[0069] Two electrolytic copper foils (each with a thickness of 18 μm)were used having different surface roughness Ra values. Otherwise, theprocedure of Experiment 1 was followed to deposit, in the form of a thinfilm having a thickness of 2 μm, an Sn—Co alloy on each electrolyticcopper foil to thereby fabricate electrodes.

[0070] The electrodes fabricated using the electrolytic copper foilshaving surface roughness Ra values of 0.188 μm and 1.19 μm weredesignated as electrodes d1 and d2 of the present invention. Likewise, athin film having a thickness of 2 μm and composed of an Sn—Co alloy wasdeposited on a rolled copper foil having a surface roughness Ra of 0.04μm to fabricate an electrode d3 of the present invention. The electroded1 of the present invention corresponds to the electrode a1 listed inTable 1.

[0071] Using each of the electrodes d1, d2 and d3 of the presentinvention, a beaker cell was constructed in the same manner as inExperiment 1, and then evaluated for charge-discharge cyclecharacteristics. The evaluation results are listed in Table 5. TABLE 51st-Cycle 1st-Cycle 10th-Cycle Capacity Charge Discharge InitialDischarge Retention Capacity Capacity Efficiency Capacity Rate Electrode(mAh/g) (mAh/g) (%) (mAh/g) (%) Electrode 772 632 82 628 99 d1 of thisInvention (Ra = 0.188) Electrode 788 656 83 620 95 d2 of this Invention(Ra = 1.19) Electrode 780 621 80 425 68 d3 of this Invention (Ra = 0.04)

[0072] As can be appreciated from the results shown in Table 5, thesurface roughness Ra of the current collector is preferably up to 1 μm,although satisfactory cycle characteristics result even when it exceeds1 μm. Improved cycle performance relative to the electrode d3 of thepresent invention is obtained for the electrode d1 of the presentinvention. These demonstrate that the particularly preferred range ofsurface roughness Ra of the current collector is 0.1-1 μm.

[0073] Experiment 5

[0074] Analogous to Experiment 1, a thin film having a thickness of 2 μmand composed of an Sn—Ni—Co alloy was deposited on an 18 μm thick,electrolytic copper foil (surface roughness Ra=0.188 μm) by anelectrolytic plating process.

[0075] Deposition of the Sn—Ni—Co alloy thin film was effected using anSn—Ni—Co plating bath containing a mixture of potassium pyrophosphate,tin chloride, nickel chloride and cobalt chloride.

[0076] Using the resulting electrode f1 of the present invention, abeaker cell was constructed in the same manner as in Experiment 1 andthen evaluated for charge-discharge cycle characteristics. Theevaluation results are given in Table 6. Also, the chemical compositionof the film deposited to form the electrode is shown in Table 7. TABLE 61st-Cycle 1st-Cycle 10th-Cycle Capacity Charge Discharge InitialDischarge Retention Capacity Capacity Efficiency Capacity Rate Electrode(mAh/g) (mAh/g) (%) (mAh/g) (%) Electrode 549 476 87 473 99 f1 of thisInvention (Sn—Ni— Co)

[0077] TABLE 7 Electrode Weight Ratio (%) Atomic Ratio (%) Electrode f1of this 72Sn-8Ni-20Co 56Sn-13Ni-31Co Invention (Sn—Ni—Co)

[0078] As apparent from the results shown in Table 6, the electrode f1incorporating the Sn—Ni—Co alloy thin film has high charge and dischargecapacities and exhibits the improved cycle characteristics relative tothe electrode c1 incorporating the Sn—Ni alloy thin film.

[0079] Utility in Industry

[0080] In accordance with the present invention, a rechargeable lithiumbattery can be provided which exhibits a high discharge capacity andimproved cycle performance characteristics.

1. (Amended) An electrode for a rechargeable lithium battery which has acurrent collector and a thin alloy film provided on the currentcollector and composed of a metal which alloys with lithium and a metalwhich does not alloy with lithium, said electrode being characterized inthat said current collector has a surface roughness Ra of 0.1 μm orlarger, said thin alloy film has a surface with irregularitiescorresponding to those defined on a surface of the current collector,and the thin alloy film is separated into islands by gaps formed, on thefirst or subsequent cycle of charge and discharge, along lines extendingin a thickness direction of the thin alloy film between valleys of theirregularities on the thin alloy film surface and on the currentcollector surface.
 2. The electrode for a rechargeable lithium batteryas recited in claim 1, characterized in that the metal which alloys withlithium is of the type that forms an intermetallic compound with themetal which does not alloy with lithium.
 3. The electrode for arechargeable lithium battery as recited in claim 1 or 2, characterizedin that said thin alloy film is deposited on the current collector by aplating process.
 4. (Deleted)
 5. (Deleted)
 6. (Deleted)
 7. (Amended) Theelectrode for a rechargeable lithium battery as recited in any one ofclaims 1-3, characterized in that said current collector has a surfaceroughness Ra of 0.1-2 μm.
 8. (Deleted)
 9. (Amended) The electrode for arechargeable lithium battery as recited in any one of claims 1-3,characterized in that said current collector has a surface roughness Raof 0.1-1 μm.
 10. The electrode for a rechargeable lithium battery asrecited in any one of claims 1-9, characterized in that said currentcollector is composed of copper.
 11. The electrode for a rechargeablelithium battery as recited in any one of claims 1-10, characterized inthat said current collector comprises an electrolytic copper foil. 12.The electrode for a rechargeable lithium battery as recited in any oneof claims 1-11, characterized in that the metal which alloys withlithium is Sn and the metal which does not alloy with lithium is of thetype that forms an intermetallic compound with Sn.
 13. The electrode fora rechargeable lithium battery as recited in claim 12, characterized inthat the metal which alloys with lithium is Sn and the metal which doesnot alloy with lithium contains at least one type selected from Fe, Coand Ni.
 14. The electrode for a rechargeable lithium battery as recitedin any one of claims 1-12, characterized in that the metal which alloyswith lithium is Sn, and at least Co is contained as the metal which doesnot alloy with lithium.
 15. The electrode for a rechargeable lithiumbattery as recited in claim 14, characterized in that the metal whichalloys with lithium is Sn, the metal which does not alloy with lithiumis Co, and said thin alloy film comprises an Sn—Co alloy.
 16. Theelectrode for a rechargeable lithium battery as recited in claim 14,characterized in that the metal which alloys with lithium is Sn, themetal which does not alloy with lithium contains Ni and Co, and saidthin alloy film comprises an Sn—Ni—Co alloy.
 17. The electrode for arechargeable lithium battery as recited in any one of claims 1-16,characterized in that a mixed layer of components of said currentcollector and thin alloy film is formed at an interface therebetween.18. A rechargeable lithium battery including a negative electrodecomprised of the electrode as recited in any one of claims 1-17, apositive electrode and a nonaqueous electrolyte.